String motion sensors, commonly known as pickups, are installed on guitars, bass guitars, mandolins and other stringed musical instruments to convert the sound produced by the vibrating strings to an electronic signal. In various applications, the electronic signal generated by a pickup may be modified using analog and digital signal processing techniques, amplified, and recorded on a suitable sound recording medium before being converted back to a sound signal by a speaker or other output transducer. Conventional musical instrument pickups use different physical principles, including variations in magnetic reluctance, the Hall effect, and the piezoelectric effect, to detect the motion of ferromagnetic strings.
Magnetic reluctance pickups typically comprise one or more ferromagnetic pole pieces, at least one magnetic flux source that generates a magnetic flux in the pole pieces, and a coil with two or more output terminals that surrounds the pole pieces. In some designs, the pole pieces generate their own flux and are formed from hard ferromagnetic materials that are magnetized as permanent magnets. In alternative designs the flux is generated by permanent magnets that are external to the pole pieces.
When the pickup is positioned near the ferromagnetic strings of a musical instrument the pole pieces induce a magnetic flux in the strings. The magnetic fluxes in the source, pole pieces and strings are mutually dependent and the magnetic flux in one or more of the pole pieces varies with string vibration. The coil surrounding the pole pieces links the flux in the pole pieces and an electromotive force is developed in the coil when the flux in the poles varies in response to string vibration. An electronic signal is developed at the output terminals of the coil in response to the electromotive force.
The frequency-dependent response function of a magnetic musical pickup is nonlinear and the audio frequency spectrum of a string's acoustic vibration is typically distorted by the pickup in the process of converting it to an electronic signal. This distortion is commonly referred to as the ‘tone’ of the pickup and, when properly controlled, adds desirable musical qualities to the output signal.
Magnetic musical instrument pickups may classified into broad categories that reflect differences in basic design and tonal quality. Pickups in the ‘single coil’ category have key design features that are shared by the pickups disclosed in U.S. Pat. No. 2,612,072 issued to H. de Armond on Sep. 30, 1952, U.S. Pat. No. 2,573,254, U.S. Pat. No. 2,817,261, U.S. Pat. No. 3,236,930, and U.S. Pat. No. 4,220,069 respectively issued to Leo Fender on Oct. 30, 1951, Dec. 24, 1957, Feb. 22, 1966, and Sep. 2, 1980 and U.S. Pat. No. 2,911,871 issued to C. F. Schultz on Nov. 10, 1959. The ‘single coil’ name derives from the fact that pickups in this category comprise a set of string-sensing ferromagnetic pole pieces with a magnetic flux that is linked by a single, string-sensing coil of wire. In some single coil pickups, the pole pieces are formed from magnetized hard ferromagnetic materials that generate the magnetic flux in the pickup. In other single coil designs one or more external permanent magnets are coupled to soft ferromagnetic pole pieces. Conventional single coil pickups have no means for external noise rejection and are sensitive to external electromagnetic noise sources.
The noise sensitivity of a pickup in which the pole pieces that sense the motion of a single string are surrounded by only one coil may be advantageously reduced by separating the pole pieces into two subsets (typically with equal numbers of poles) and surrounding each subset of poles with a different coil. By reversing the magnetic polarity of each subset of poles and causing the signal to traverse the coils in opposite directions, string-generated signals from the two coils may be summed and the noise-generated signals at least partially cancelled. Commercial examples of split coil pickups that employ this approach include the P-Bass pickup that is disclosed in U.S. Pat. No. 2,976,755 issued to C. L. Fender on Jan. 6, 1959 and Z-coil pickup that is manufactured and installed as original equipment on the Comanche model six string guitars manufactured by G&L Guitar Company of Fullerton, Calif.
Noise-reducing humbucking pickups or ‘humbuckers’ share key design features with the devices that are disclosed in U.S. Pat. No. 2,896,491 ('491) issued to Seth Lover on Jul. 28, 1959, U.S. Pat. No. 4,220,069 ('069) issued to C. Leo Fender on Sep. 2, 1980, and U.S. Pat. No. 2,892,371 ('371) issued to J. R. Butts on Jun. 30, 1959. Pickups in this class have at least two-string sensing coils, each linked to a separate set of string-sensing pole pieces. The magnetic field direction in the poles and the direction of signal propagation within the coils are selected so that a large portion of the string-generated signals from the two are coils have an in-phase, additive relationship and a large percentage of the common-mode noise signals from the two coils have an out-of-phase, subtractive relationship. In most cases, the amplitude of the output signal of a humbucking pickup is greater than that obtained from a single coil pickup and the output noise signal is significantly reduced.
Magnetic pickups have been developed for a wide variety of musical instruments. In addition to six string guitars, magnetic pickups are commonly used on other lute-type stringed instruments such as 12-string guitars, bass guitars, mandolins, and steel guitar. Magnetic pickups have also been developed for percussion instruments including marimbas, xylophones and pianos. The scope of the present invention is limited to lute-type instruments with ferromagnetic strings and, for purposes of clarity, the features of the present invention will be discussed with reference to a 6-string guitar. Those skilled in the art will, however, realize that the scope of the invention is not limited to the exemplary six string guitars with ferromagnetic strings but extends to a wide range of stringed musical instruments.
The design and manufacture of magnetic musical instrument pickups are described from an historical and lay engineering perspective in The Guitar Pickup Handbook, the Start of Your Sound by Duncan Hunter (Backbeat/Hal Leonard, New York, 2008) and Pickups, Windings and Magnets and the Guitar Became Electric, by Mario Milan (Centerstream, Anaheim Hills, 2007). On a more technical level, Engineering the Guitar, Theory and Practice by Richard Mark French (Springer, New York, 2009) contains a chapter on Guitar Electronics and a thorough treatment of musical sound quality and tone as viewed from an engineering and physics perspective.